Abstract Body (Do not enter title and authors here): In the adult heart, the epicardium is activated following myocardial infarction (MI), contributing to cardiac repair primarily through the secretion of paracrine factors. However, the specific cellular subtypes and molecular signaling pathways that govern epicardial function during injury remain poorly understood. This study investigates the dynamic role of the epicardium in modulating the cardiac response to MI.

Using the Wt1 (Wilms Tumor 1)^CreERT2; R26^tdTomato tamoxifen-inducible epicardial-specific lineage tracing mouse model in combination with a Pdgfra^nGFP fibroblast reporter, we performed single-cell RNA sequencing on epicardial-derived cells (tdTomato⁺) isolated from sham, 7-, and 14-day post-MI hearts. We identified 9 transcriptionally distinct epicardial subpopulations, which clustered into three major functional groups with distinct roles in cardiac remodeling. Wt1⁺ Pdgfra⁺ epicardial fibroblast-like cells were enriched for chemokines and semaphorins, suggesting a role in immune cell recruitment and vascular remodeling. In contrast, Wt1⁺, Upk3b⁺, Msln⁺ epicardial mesothelial-like cells showed enrichment in Wnt signaling and semaphorins. Temporal analysis revealed that epicardial cells actively regulate low-grade inflammation and leukocyte recruitment throughout the 7- to 14-day post-MI period.

Altogether, our study highlights the heterogeneous and dynamic nature of epicardial cell function during cardiac repair and underscore the epicardium's critical role in orchestrating myocardial remodeling following injury.